Seasonal biology: Tanycytes give the hypothalamus a spring makeover.

In many species, metabolic and reproductive functions are coupled to the seasons. Tanycytes, specialized glial cells in the hypothalamus, play an important function in these physiological changes. A new study now shows that light exposure drastically alters the formation of sensory cilia on tanycytes.

A brainstem-hypothalamus neuronal circuit reduces feeding upon heat exposure.

Empirical evidence suggests that heat exposure reduces food intake. However, the neurocircuit architecture and the signalling mechanisms that form an associative interface between sensory and metabolic modalities remain unknown, despite primary thermoceptive neurons in the pontine parabrachial nucleus becoming well characterized1. Tanycytes are a specialized cell type along the wall of the third ventricle2 that bidirectionally transport hormones and signalling molecules between the brain's parenchyma and ventricular system3-8. Here we show that tanycytes are activated upon acute thermal challenge and are necessary to reduce food intake afterwards. Virus-mediated gene manipulation and circuit mapping showed that thermosensing glutamatergic neurons of the parabrachial nucleus innervate tanycytes either directly or through second-order hypothalamic neurons. Heat-dependent Fos expression in tanycytes suggested their ability to produce signalling molecules, including vascular endothelial growth factor A (VEGFA). Instead of discharging VEGFA into the cerebrospinal fluid for a systemic effect, VEGFA was released along the parenchymal processes of tanycytes in the arcuate nucleus. VEGFA then increased the spike threshold of Flt1-expressing dopamine and agouti-related peptide (Agrp)-containing neurons, thus priming net anorexigenic output. Indeed, both acute heat and the chemogenetic activation of glutamatergic parabrachial neurons at thermoneutrality reduced food intake for hours, in a manner that is sensitive to both Vegfa loss-of-function and blockage of vesicle-associated membrane protein 2 (VAMP2)-dependent exocytosis from tanycytes. Overall, we define a multimodal neurocircuit in which tanycytes link parabrachial sensory relay to the long-term enforcement of a metabolic code.

Cell proliferation and glial cell marker expression in the wall of the third ventricle in the tuberal region of the male mouse hypothalamus during postnatal development.

The third ventricle (3 V) wall of the tuberal hypothalamus is composed of two types of cells; specialized ependymoglial cells called tanycytes located ventrally and ependymocytes dorsally, which control the exchanges between the cerebrospinal fluid and the hypothalamic parenchyma. By regulating the dialogue between the brain and the periphery, tanycytes are now recognized as central players in the control of major hypothalamic functions such as energy metabolism and reproduction. While our knowledge of the biology of adult tanycytes is progressing rapidly, our understanding of their development remains very incomplete. To gain insight into the postnatal maturation of the 3 V ependymal lining, we conducted a comprehensive immunofluorescent study of the mouse tuberal region at four postnatal ages (postnatal day (P) 0, P4, P10, and P20). We analyzed the expression profile of a panel of tanycyte and ependymocyte markers (vimentin, S100, connexin-43 [Cx43], and glial fibrillary acidic protein [GFAP]) and characterized cell proliferation in the 3 V wall using the thymidine analog bromodeoxyuridine. Our results show that most changes in marker expression occur between P4 and P10, with a switch from a 3 V mostly lined by radial cells to the emergence of a tanycytic domain ventrally and an ependymocytic domain dorsally, a drop in cell proliferation and increased expression of S100, Cx43, and GFAP that acquire a mature profile at P20. Our study thus identifies the transition between the first and the second postnatal week as a critical time window for the postnatal maturation of the 3 V wall ependymal lining.

Male minipuberty involves the gonad-independent activation of preoptic nNOS neurons.

BACKGROUND: The maturation of the hypothalamic-pituitary-gonadal (HPG) axis is crucial for the establishment of reproductive function. In female mice, neuronal nitric oxide synthase (nNOS) activity appears to be key for the first postnatal activation of the neural network promoting the release of gonadotropin-releasing hormone (GnRH), i.e. minipuberty. However, in males, the profile of minipuberty as well as the role of nNOS-expressing neurons remain unexplored. METHODS: nNOS-deficient and wild-type mice were studied during postnatal development. The expression of androgen (AR) and estrogen receptor alpha (ERα) as well as nNOS phosphorylation were evaluated by immunohistochemistry in nNOS neurons in the median preoptic nucleus (MePO), where most GnRH neuronal cell bodies reside, and the hormonal profile of nNOS-deficient male mice was assessed using previously established radioimmunoassay and ELISA methods. Gonadectomy and pharmacological manipulation of ERα were used to elucidate the mechanism of minipubertal nNOS activation and the maturation of the HPG axis. RESULTS: In male mice, minipubertal FSH release occurred at P23, preceding the LH surge at P30, when balanopreputial separation occurs. Progesterone and testosterone remained low during minipuberty, increasing around puberty, whereas estrogen levels were high throughout postnatal development. nNOS neurons showed a sharp increase in Ser1412 phosphorylation of nNOS at P23, a phenomenon that occurred even in the absence of the gonads. In male mice, nNOS neurons did not appear to express AR, but abundantly expressed ERα throughout postnatal development. Selective pharmacological blockade of ERα during the infantile period blunted Ser1412 phosphorylation of nNOS at P23. CONCLUSIONS: Our results show that the timing of minipuberty differs in male mice when compared to females, but as in the latter, nNOS activity in the preoptic region plays a role in this process. Additionally, akin to male non-human primates, the profile of minipuberty in male mice is shaped by sex-independent mechanisms, and possibly involves extragonadal estrogen sources.

GnRH replacement rescues cognition in Down syndrome.

At the present time, no viable treatment exists for cognitive and olfactory deficits in Down syndrome (DS). We show in a DS model (Ts65Dn mice) that these progressive nonreproductive neurological symptoms closely parallel a postpubertal decrease in hypothalamic as well as extrahypothalamic expression of a master molecule that controls reproduction-gonadotropin-releasing hormone (GnRH)-and appear related to an imbalance in a microRNA-gene network known to regulate GnRH neuron maturation together with altered hippocampal synaptic transmission. Epigenetic, cellular, chemogenetic, and pharmacological interventions that restore physiological GnRH levels abolish olfactory and cognitive defects in Ts65Dn mice, whereas pulsatile GnRH therapy improves cognition and brain connectivity in adult DS patients. GnRH thus plays a crucial role in olfaction and cognition, and pulsatile GnRH therapy holds promise to improve cognitive deficits in DS.

Prolonged breastfeeding protects from obesity by hypothalamic action of hepatic FGF21.

Early-life determinants are thought to be a major factor in the rapid increase of obesity. However, while maternal nutrition has been extensively studied, the effects of breastfeeding by the infant on the reprogramming of energy balance in childhood and throughout adulthood remain largely unknown. Here we show that delayed weaning in rat pups protects them against diet-induced obesity in adulthood, through enhanced brown adipose tissue thermogenesis and energy expenditure. In-depth metabolic phenotyping in this rat model as well as in transgenic mice reveals that the effects of prolonged suckling are mediated by increased hepatic fibroblast growth factor 21 (FGF21) production and tanycyte-controlled access to the hypothalamus in adulthood. Specifically, FGF21 activates GABA-containing neurons expressing dopamine receptor 2 in the lateral hypothalamic area and zona incerta. Prolonged breastfeeding thus constitutes a protective mechanism against obesity by affecting long-lasting physiological changes in liver-to-hypothalamus communication and hypothalamic metabolic regulation.

Tanycytes control hypothalamic liraglutide uptake and its anti-obesity actions.

Liraglutide, an anti-diabetic drug and agonist of the glucagon-like peptide one receptor (GLP1R), has recently been approved to treat obesity in individuals with or without type 2 diabetes. Despite its extensive metabolic benefits, the mechanism and site of action of liraglutide remain unclear. Here, we demonstrate that liraglutide is shuttled to target cells in the mouse hypothalamus by specialized ependymoglial cells called tanycytes, bypassing the blood-brain barrier. Selectively silencing GLP1R in tanycytes or inhibiting tanycytic transcytosis by botulinum neurotoxin expression not only hampers liraglutide transport into the brain and its activation of target hypothalamic neurons, but also blocks its anti-obesity effects on food intake, body weight and fat mass, and fatty acid oxidation. Collectively, these striking data indicate that the liraglutide-induced activation of hypothalamic neurons and its downstream metabolic effects are mediated by its tanycytic transport into the mediobasal hypothalamus, strengthening the notion of tanycytes as key regulators of metabolic homeostasis.

The polygamous GnRH neuron: Astrocytic and tanycytic communication with a neuroendocrine neuronal population.

To ensure the survival of the species, hypothalamic neuroendocrine circuits controlling fertility, which converge onto neurons producing gonadotropin-releasing hormone (GnRH), must respond to fluctuating physiological conditions by undergoing rapid and reversible structural and functional changes. However, GnRH neurons do not act alone, but through reciprocal interactions with multiple hypothalamic cell populations, including several glial and endothelial cell types. For instance, it has long been known that in the hypothalamic median eminence, where GnRH axons terminate and release their neurohormone into the pituitary portal blood circulation, morphological plasticity displayed by distal processes of tanycytes modifies their relationship with adjacent neurons as well as the spatial properties of the neurohemal junction. These alterations not only regulate the capacity of GnRH neurons to release their neurohormone, but also the activation of discrete non-neuronal pathways that mediate feedback by peripheral hormones onto the hypothalamus. Additionally, a recent breakthrough has demonstrated that GnRH neurons themselves orchestrate the establishment of their neuroendocrine circuitry during postnatal development by recruiting an entourage of newborn astrocytes that escort them into adulthood and, via signalling through gliotransmitters such as prostaglandin E2, modulate their activity and GnRH release. Intriguingly, several environmental and behavioural toxins perturb these neuron-glia interactions and consequently, reproductive maturation and fertility. Deciphering the communication between GnRH neurons and other neural cell types constituting hypothalamic neuroendocrine circuits is thus critical both to understanding physiological processes such as puberty, oestrous cyclicity and aging, and to developing novel therapeutic strategies for dysfunctions of these processes, including the effects of endocrine disruptors.

GnRH neurons recruit astrocytes in infancy to facilitate network integration and sexual maturation.

Neurons that produce gonadotropin-releasing hormone (GnRH), which control fertility, complete their nose-to-brain migration by birth. However, their function depends on integration within a complex neuroglial network during postnatal development. Here, we show that rodent GnRH neurons use a prostaglandin D2 receptor DP1 signaling mechanism during infancy to recruit newborn astrocytes that 'escort' them into adulthood, and that the impairment of postnatal hypothalamic gliogenesis markedly alters sexual maturation by preventing this recruitment, a process mimicked by the endocrine disruptor bisphenol A. Inhibition of DP1 signaling in the infantile preoptic region, where GnRH cell bodies reside, disrupts the correct wiring and firing of GnRH neurons, alters minipuberty or the first activation of the hypothalamic-pituitary-gonadal axis during infancy, and delays the timely acquisition of reproductive capacity. These findings uncover a previously unknown neuron-to-neural-progenitor communication pathway and demonstrate that postnatal astrogenesis is a basic component of a complex set of mechanisms used by the neuroendocrine brain to control sexual maturation.

Tanycytes in the infundibular nucleus and median eminence and their role in the blood-brain barrier.

The blood-brain barrier is generally attributed to endothelial cells. However, in circumventricular organs, such as the median eminence, tanycytes take over the barrier function. These ependymoglial cells form the wall of the third ventricle and send long extensions into the parenchyma to contact blood vessels and hypothalamic neurons. The shape and location of tanycytes put them in an ideal position to connect the periphery with central nervous compartments. In line with this, tanycytes control the transport of hormones and key metabolites in and out of the hypothalamus. They function as sensors of peripheral homeostasis for central regulatory networks. This chapter discusses current evidence that tanycytes play a key role in regulating glucose balance, food intake, endocrine axes, seasonal changes, reproductive function, and aging. The understanding of how tanycytes perform these diverse tasks is only just beginning to emerge and will probably lead to a more differentiated view of how the brain and the periphery interact.

The KiNG of reproduction: Kisspeptin/ nNOS interactions shaping hypothalamic GnRH release.

Gonadotropin-releasing hormone (GnRH) is the master regulator of the hypothalamic-pituitary-gonadal (HPG) axis, and therefore of fertility and reproduction. The release pattern of GnRH by the hypothalamus includes both pulses and surges. However, despite a considerable body of evidence in support of a determinant role for kisspeptin, the mechanisms regulating a GnRH pulse and surge remain a topic of debate. In this review we challenge the view of kisspeptin as an absolute "monarch", and instead present the idea of a Kisspeptin-nNOS-GnRH or "KiNG" network that is responsible for generating the "GnRH pulse" and "GnRH surge". In particular, the neuromodulator nitric oxide (NO) has opposite effects to kisspeptin on GnRH secretion in many respects, acting as the Yin to kisspeptin's Yang and creating a dynamic system in which kisspeptin provides the "ON" signal, promoting GnRH release, while NO mediates the "OFF" signal, acting as a tonic brake on GnRH secretion. This interplay between an activator and an inhibitor, which is in turn fine-tuned by the gonadal steroid environment, thus leads to the generation of GnRH pulses and surges and is crucial for the proper development and function of the reproductive axis.

Hypothalamic bile acid-TGR5 signaling protects from obesity.

Bile acids (BAs) improve metabolism and exert anti-obesity effects through the activation of the Takeda G protein-coupled receptor 5 (TGR5) in peripheral tissues. TGR5 is also found in the brain hypothalamus, but whether hypothalamic BA signaling is implicated in body weight control and obesity pathophysiology remains unknown. Here we show that hypothalamic BA content is reduced in diet-induced obese mice. Central administration of BAs or a specific TGR5 agonist in these animals decreases body weight and fat mass by activating the sympathetic nervous system, thereby promoting negative energy balance. Conversely, genetic downregulation of hypothalamic TGR5 expression in the mediobasal hypothalamus favors the development of obesity and worsens established obesity by blunting sympathetic activity. Lastly, hypothalamic TGR5 signaling is required for the anti-obesity action of dietary BA supplementation. Together, these findings identify hypothalamic TGR5 signaling as a key mediator of a top-down neural mechanism that counteracts diet-induced obesity.

[Precocious puberty and neuropilin-1 signaling in GnRH neurons]. / Signalisation impliquant la neuropiline dans les neurones sécrétant la GnRH - Son rôle dans le déclenchement de la puberté.

The survival of the species depends on two closely interlinked processes: the correct functioning of the reproductive system, and the balance between the energy needs of an individual and the supply of energy sources through feeding. These two processes are regulated in the hypothalamus, which produces neurohormones that control various physiological functions. Among these neurohormones, GnRH controls not only the maturation and function of the reproductive organs, including the ovaries and the testes, during puberty and in adulthood, but also sexual attraction. Recent evidence suggest that neuropilin-1-mediated signaling in GnRH-synthesizing neurons could be a linchpin that holds together various neuroanatomical, physiological and behavioral adaptations involved in triggering puberty and achieving reproductive function.

TITLE: Signalisation impliquant la neuropiline dans les neurones sécrétant la GnRH - Son rôle dans le déclenchement de la puberté. ABSTRACT: La survie d'une espèce dépend de deux processus intimement liés : la reproduction, d'une part, et l'équilibre entre les besoins énergétiques et l'approvisionnement en sources d'énergie par l'alimentation, d'autre part. Ces deux processus sont contrôlés dans le cerveau par l'hypothalamus, qui produit des neurohormones agissant sur l'hypophyse pour piloter diverses fonctions physiologiques. L'une de ces neurohormones, la GnRH, contrôle non seulement la maturation et le fonctionnement des organes reproducteurs, incluant les ovaires et les testicules, lors de la puberté et à l'âge adulte, mais aussi l'attirance sexuelle. De récentes découvertes suggèrent que la signalisation impliquant la neuropiline-1 dans les neurones sécrétant la GnRH jouerait un rôle charnière dans la coordination du neurodéveloppement et des adaptations physiologiques et comportementales nécessaires au déclenchement de la puberté et à l'acquisition de la fonction de reproduction. Dans cet article de synthèse, nous replaçons ces découvertes dans le contexte de récents travaux montrant que les voies de signalisation des sémaphorines de classe 3 sont impliquées dans la physiopathologie non seulement de l'infertilité, mais aussi de l'obésité. Nous discutons également l'implication potentielle des neurones produisant la GnRH dans la perception des odeurs sociales et dans la précocité de la maturation sexuelle. L'hypothèse selon laquelle l'activité de ces neurones au cours du développement postnatal constituerait le chaînon manquant entre la prise de poids, le déclenchement de la puberté et le comportement sexuel, ouvre la voie à une meilleure compréhension de l'implication de l'homéostasie énergétique dans la maturation sexuelle, et pourrait aussi avoir des implications thérapeutiques pour la puberté précoce.

Glial endozepines and energy balance: Old peptides with new tricks.

The contribution of neuroglial interactions to the regulation of energy balance has gained increasing acceptance in recent years. In this context, endozepines, endogenous analogs of benzodiazepine derived from diazepam-binding inhibitor, are now emerging as major players. Produced by glial cells (astrocytes and tanycytes), endozepines have been known for two decades to exert potent anorexigenic effects by acting at the hypothalamic level. However, it is only recently that their modes of action, including the mechanisms by which they modulate energy metabolism, have begun to be elucidated. The data available today are abundant, significant, and sometimes contradictory, revealing a much more complex regulation than initially expected. Several mechanisms of action of endozepines seem to coexist at the central level, particularly in the hypothalamus. The brainstem has also recently emerged as a potential site of action for endozepines. In addition to their central anorexigenic effects, endozepines may also display peripheral effects promoting orexigenic actions, adding to their complexity and raising yet more questions. In this review, we attempt to provide an overview of our current knowledge in this rapidly evolving field and to pinpoint questions that remain unanswered.

Glial Endozepines Reverse High-Fat Diet-Induced Obesity by Enhancing Hypothalamic Response to Peripheral Leptin.

Research on energy homeostasis has focused on neuronal signaling; however, the role of glial cells has remained little explored. Glial endozepines exert anorexigenic actions by mechanisms which remain poorly understood. In this context, the present study was designed to decipher the mechanisms underlying the anorexigenic action of endozepines and to investigate their potential curative effect on high-fat diet-induced obesity. We carried out a combination of physiological, pharmacological, and molecular analyses together to dissect the underlying mechanisms of endozepine-induced hypophagia. To evaluate the potential anti-obesity effect of endozepines, different model of obesity were used, i.e., ob/ob and diet-induced obese mice. We show that the intracerebral administration of endozepines enhances satiety by targeting anorexigenic brain circuitry and induces STAT3 phosphorylation, a hallmark of leptin signaling. Strikingly, endozepines are entirely ineffective at reducing food intake in the presence of a circulating leptin antagonist and in leptin-deficient mice (ob/ob) but potentiate the reduced food intake and weight loss induced by exogenous leptin administration in these animals. Endozepines reversed high fat diet-induced obesity by reducing food intake and restored leptin-induced STAT3 phosphorylation in the hypothalamus. Interestingly, we observed that glucose and insulin synergistically enhance tanycytic endozepine expression and release. Finally, endozepines, which induce ERK activation necessary for leptin transport into the brain in cultured tanycytes, require tanycytic leptin receptor expression to promote STAT3 phosphorylation in the hypothalamus. Our data identify endozepines as potential anti-obesity compounds in part through the modulation of the LepR-ERK-dependent tanycytic leptin shuttle.

Hypothalamic miR-30 regulates puberty onset via repression of the puberty-suppressing factor, Mkrn3.

Mkrn3, the maternally imprinted gene encoding the makorin RING-finger protein-3, has recently emerged as putative pubertal repressor, as evidenced by central precocity caused by MKRN3 mutations in humans; yet, the molecular underpinnings of this key regulatory action remain largely unexplored. We report herein that the microRNA, miR-30, with three binding sites in a highly conserved region of its 3' UTR, operates as repressor of Mkrn3 to control pubertal onset. Hypothalamic miR-30b expression increased, while Mkrn3 mRNA and protein content decreased, during rat postnatal maturation. Neonatal estrogen exposure, causing pubertal alterations, enhanced hypothalamic Mkrn3 and suppressed miR-30b expression in female rats. Functional in vitro analyses demonstrated a strong repressive action of miR-30b on Mkrn3 3' UTR. Moreover, central infusion during the juvenile period of target site blockers, tailored to prevent miR-30 binding to Mkrn3 3' UTR, reversed the prepubertal down-regulation of hypothalamic Mkrn3 protein and delayed female puberty. Collectively, our data unveil a novel hypothalamic miRNA pathway, involving miR-30, with a prominent role in the control of puberty via Mkrn3 repression. These findings expand our current understanding of the molecular basis of puberty and its disease states.

Hypothalamic dopamine signaling regulates brown fat thermogenesis.

Dopamine signaling is a crucial part of the brain reward system and can affect feeding behavior. Dopamine receptors are also expressed in the hypothalamus, which is known to control energy metabolism in peripheral tissues. Here we show that pharmacological or chemogenetic stimulation of dopamine receptor 2 (D2R) expressing cells in the lateral hypothalamic area (LHA) and the zona incerta (ZI) decreases body weight and stimulates brown fat activity in rodents in a feeding-independent manner. LHA/ZI D2R stimulation requires an intact sympathetic nervous system and orexin system to exert its action and involves inhibition of PI3K in the LHA/ZI. We further demonstrate that, as early as 3 months after onset of treatment, patients treated with the D2R agonist cabergoline experience an increase in energy expenditure that persists for one year, leading to total body weight and fat loss through a prolactin-independent mechanism. Our results may provide a mechanistic explanation for how clinically used D2R agonists act in the CNS to regulate energy balance.

MCH Regulates SIRT1/FoxO1 and Reduces POMC Neuronal Activity to Induce Hyperphagia, Adiposity, and Glucose Intolerance.

Melanin-concentrating hormone (MCH) is an important regulator of food intake, glucose metabolism, and adiposity. However, the mechanisms mediating these actions remain largely unknown. We used pharmacological and genetic approaches to show that the sirtuin 1 (SIRT1)/FoxO1 signaling pathway in the hypothalamic arcuate nucleus (ARC) mediates MCH-induced feeding, adiposity, and glucose intolerance. MCH reduces proopiomelanocortin (POMC) neuronal activity, and the SIRT1/FoxO1 pathway regulates the inhibitory effect of MCH on POMC expression. Remarkably, the metabolic actions of MCH are compromised in mice lacking SIRT1 specifically in POMC neurons. Of note, the actions of MCH are independent of agouti-related peptide (AgRP) neurons because inhibition of γ-aminobutyric acid receptor in the ARC did not prevent the orexigenic action of MCH, and the hypophagic effect of MCH silencing was maintained after chemogenetic stimulation of AgRP neurons. Central SIRT1 is required for MCH-induced weight gain through its actions on the sympathetic nervous system. The central MCH knockdown causes hypophagia and weight loss in diet-induced obese wild-type mice; however, these effects were abolished in mice overexpressing SIRT1 fed a high-fat diet. These data reveal the neuronal basis for the effects of MCH on food intake, body weight, and glucose metabolism and highlight the relevance of SIRT1/FoxO1 pathway in obesity.